Use multiselects for the recipient selection

css/mail.scss

lib/Service/AutoCompletion/AutoCompleteService.php

lib/Service/ContactsIntegration.php

lib/Service/GroupsIntegration.php

package-lock.json

package.json

src/components/Address.vue

src/components/Composer.vue

src/service/AutocompleteService.js

src/store.js

src/tests/unit/components/Address.spec.js

tests/Service/Autocompletion/AutoCompleteServiceTest.php

tests/Service/ContactsIntegrationTest.php

tests/Service/GroupsIntegrationTest.php